Phase Transitions of Liquid Crystal PAA in Confined Geometries

Phase transitions of liquid crystal PAA, p-azoxyanisole, in porous sol-gels of average pore diameters 8.9,. 16, and 20 nm, are investigated using Rama...
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J. Phys. Chem. B 1998, 102, 3387-3394

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Phase Transitions of Liquid Crystal PAA in Confined Geometries Y. Shao and T. W. Zerda* Physics Department, TCU Box 298840, Fort Worth, Texas 76129 ReceiVed: October 24, 1997; In Final Form: March 3, 1998

Phase transitions of liquid crystal PAA, p-azoxyanisole, in porous sol-gels of average pore diameters 8.9, 16, and 20 nm, are investigated using Raman spectroscopy and differential scanning calorimetry. It is found that the phase transition temperatures inside small pores are depressed and the depressions are pore size dependent. This phenomenon is observed for the solid to nematic and the nematic to isotropic phase transitions of PAA. Surface modifications in which -OH groups are replaced by -Si(CH)3 groups are not sufficient to modify the phase behavior. The restricted space limits the size of a crystal and dominates the phase transitions. Confinement results in broadening of the phase transitions, indicating that in the pores they become weak first-order transitions.

Introduction Molecules in small pores, with characteristic dimension comparable to the molecular size, usually exhibit behavior different from that in their bulk phases. The influence of geometrical restrictions on physical and chemical properties of guest molecules has attracted wide interest in the past decade. Studies on rotational and vibrational relaxation of small molecules showed that the surface conditions of the pore wall play a dominant role in restricting molecular reorientation, especially in the layer close to the pore surface.1-6 Translational molecular motion of small solvent molecules as well as large systems such as steroids, metal complexes, nanocrystallites, and liquid crystals have been studied in small pores.7,8 Freezing temperatures for simple molecular liquids have been found to take place at temperatures lower than in the bulk phase.9,10 Similar results have been obtained for phase transitions of PAA11 and 5CB,12 and other liquid crystals.13-16 The change of the type of phase transition from the first order (in the bulk) to the second order in small pores has been reported.12,17 When the cavities are well-defined and ordered, an increase of the isotropic to nematic, I-N, transition temperature has been observed and explained in terms of surface-induced order.13-15 This finding indicated that the pore diameters alone cannot explain the behavior of liquid crystal molecules in small pores. Surface interactions and other geometric effects, such as pore connectivity and regularity of the pore curvature, must be included. All the above developments in studying space confinement effects had been made possible with the help of advanced materials engineering in manufacturing monolithic porous materials, such as Nucleopore and Anopore membranes, Vycor glass, silica gels, and aerogels. In particular, silica gels, produced from a low-temperature sol-gel process,18 are chemically pure and spectroscopically transparent. These properties made silica gels widely accepted as the ideal host materials for confinement studies. Silica gels have a narrow pore size distribution, which is necessary to distinguish the pore size effects. Another advantage of using sol-gel glass is the possibility of working with very small pore diameters. The cavities have irregular shapes and are randomly connected. The * To whom correspondence should be addressed.

lack of order in the structure of the cavities prevents long-range ordering in adsorbed molecules, such as those observed in the cavities present within Anopore and Nucleopore membranes. But the pore surfaces of sol-gel glass can be easily modified, and thus it is possible to control interactions between the pore surfaces and adsorbed molecules. Most of the studies on confinement effects were limited to geometrical effects and did not address the question of surface interactions and how they might affect various phase transitions. To answer this question and to expand experimental evidence for the depression of phase transition in small geometries, we decided to do a series of studies for molecular systems in porous sol-gel glass. Sol-gels of different pore sizes and modified and unmodified surfaces were used. This paper focuses on two phase transitions of PAA, the isotropic to nematic, I-N, and the nematic to solid, N-S. The main experimental technique used in this study is Raman spectroscopy, and the supporting data are obtained from the differential scanning calorimetry (DSC) measurements. Experimental Section Samples of porous silica glass were prepared using the solgel process. The preparation method has been described elsewhere.19 Silica glass stabilized at about 1000 K has a porous structure, which offers an interconnected void space. The porosity for a typical sol-gel may reach 50%-70% of the total sample volume. Sol-gel samples were prepared in sets of about 50. Several samples from the same batch were analyzed for the pore volume, diameter, and surface area. The pore diameters were measured by the nitrogen adsorption method using a Quantachrome Autosorb-1. Three different pore size samples were used in the liquid crystal phase transition study. The gel samples with 16 nm average pore diameter had dimensions of about 5 mm × 5 mm × 3 mm. Sol-gel samples of average pore diameter 20 nm were in the shape of a disk of diameter 5.8 mm and 2.5 mm thick. The porous Vycor glass rods, purchased from Corning Glass Co., were cut into disks 15 mm in diameter and 2 mm thick. The BET characterization showed that the average pore size of Vycor glass was 8.9 nm. Prior to the measurements all samples were heated to 300 °C for 5 h to

S1089-5647(97)03443-3 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/10/1998

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Shao and Zerda

Figure 1. Raman spectra of the bulk PAA in the solid (temperatures below 391 K), nematic (temperature range 391-409 K), and the liquid (above 409 K) phases.

remove the moisture and organic impurities that might be adsorbed on the pore surface. The gels made with the process discussed above have hydroxyl groups on the pore surface. Samples fired at 1100 K have about 1 or 2 -OH groups per nm2. The -OH groups are able to form hydrogen bonds with the molecules inside pores and are highly reactive with some materials. To investigate the surface interactions between impregnated molecules and surface groups, we modified the gel pore surface by substituting hydroxyl groups with the less reactive trimethylsilyl, -Si(CH3)3, groups. We followed the procedure proposed by Sindolf and Maciel20 and used hexamethyldisilazane (HMDS) to modify the pore surface. It was shown that over 90% of hydroxyl groups were converted in the HMDS modification process.21 The chemical reaction is given by

(CH3)3SiNHSi(CH3)3 + 2SiOH f 2(CH3)3SiOSi + NH3 (1) PAA (p-azoxyanisole) was purchased from Aldrich and used without further purification. To impregnate the molecules into the pores, the glass samples together with the liquid crystal were heated to 430 K, some 20 degrees above the melting temperature for 2-24 h. The material moved into the pores due to capillary forces. Raman measurements were carried out on a SPEX Triplemate Spectrometer. The excitation source was an Ar+ laser with a wavelength of 488 nm. An Olympus BH-2 microscope was attached to the entry slit of the spectrometer and focused the laser beam onto a sample. The objective collected the Raman scattering from the sample and delivered the signal to the spectrometer. The whole setup had a 180° backscattering geometry. A grating of 1800 groves/mm was used, and a CCD detector recorded the spectrum with a resolution of 0.7 cm-1/ pixel. The laser beam was focused by the microscope onto an area of about 10 µm in diameter. Extreme heat may not be dissipated right away and causes a large temperature gradient within the sample. To avoid laser heating, the samples were spinning at a rate of about 1000 revolutions/min. Calibration runs showed that the laser heating effect was reduced to less than 1 °C. Every experiment in this study was repeated two to four times. All DSC measurements were carried out on a TA Instruments DSC-10 differential scanning calorimeter. The sample aluminum pans had inside diameters of 3 mm and were 2 mm tall. The total amount of sample loaded into the pan may directly

affect the sensitivity of the measurements. We found that 1020 mg of the sample gave the best results. The heating rate was set to 5 °C/min to achieve good reproducibility (less than (1 °C) and sensitivity. The nitrogen gas was introduced to keep moisture out and help keep a temperature balance inside the whole heating compartment. The flow rate was kept at 50 mL/min throughout an experiment. The DSC measurements were repeated for selected samples, and the data presented in this paper are the average results. Results At room temperature and normal pressure, pure PAA forms a solid phase. When the temperature is increased to 391 K, a nematic liquid crystal is formed. Above 409 K PAA changes into a liquid phase. Those phase transitions generate changes in the Raman spectra. Stacked contour plots of pure PAA in the region between 1150 and 1350 cm-1 are shown in Figure 1, and in a broader frequency range, 1000-1650 cm-1, in Figure 1s. The Raman spectrum of PAA is quite complicated. For large molecules, such as PAA, a number of intramolecular couplings may be present, including Fermi resonance, intramolecular hydrogen bonding, combination bands, phonons, and intermolecular coupling. Those interactions make band assignment difficult, but due to extensive studies of Freyman et al.,22 Maier et al.,23 Amer et al.,24 and Kirov et al.,25 the origins of the peaks are well understood. It is obvious that the phase transitions affect several band shapes. As can be seen in Figure 1, significant changes in band intensities and small frequency shifts accompany the nematic to solid phase transition, N-S, but the band shapes are almost unchanged around the isotropic to nematic transition, I-N. In Figures 2 and 2s we show Raman spectra of PAA in the pores 20, 16, and 8.9 nm in diameter. It is seen that the transitions are not as sharp as for the pure system, and most changes are associated with the N-S transition. The signal-to-noise ratio drops significantly upon loading PAA into porous glass, and we concentrated our efforts only on the intense peaks located in the region between 1150 and 1350 cm-1, Figure 2. During the melting transition, the intensities in the nematic phase are similar to those observed in the isotropic phase. The frequency shifts are very small and difficult to observe. Also changes in the bandwidths are small and within experimental error. However, for some bands the broadening is obvious; for example, the temperature dependence of the bandwidth for the benzene stretch mode at 1170 cm-1 is depicted in Figure 3 for the bulk PAA and PAA inside the pores.

Liquid Crystal PAA

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Figure 2. Raman spectra of PAA inside porous gels of various pore diameters: A, 20 nm; B, 16 nm; C, 8.9 nm.

Spectral changes during the N-S transition are more pronounced. Several peaks shift their positions by 2 or more wavenumbers; two examples are shown in Figures 4 and 5. These shifts indicate that molecular environment in the solid phase is very different from that in the nematic phase. Additional evidence for phase transitions is provided by the analysis of Raman band intensities. The contour spectra shown in Figures 1 and 2 were obtained for the same carefully controlled experimental conditions. The laser power, scattering geometry, sample position, orientation of diffraction gratings, and recording times did not change during the runs. It can be seen that intensities of several bands abruptly change when the solid phase is formed. Since Raman spectroscopy does not allow for absolute intensity measurements, to characterize those changes we calculated the intensity ratio of two bands centered at 1244 and 1272 cm-1. Because for each run, the two bands were recorded simultaneously, their intensity ratio, R ) I1244/ I1272, is a good measure of changes in the scattering intensities

in the nematic versus the solid phase. In Figure 6, R is plotted against temperature. For pure PAA, at 391 K a jump in the value of R can be clearly noticed. For PAA inside small pores the changes in R are spread over a wide range of temperatures. Using three different approaches, measurements of the band intensity ratio, the frequency shift, and the bandwidth, we can determine the phase transition point for pure PAA. The results from these different measurements are consistent within 2 °C. It is important to note that the precision of those different methods is not the same. The Raman spectrometer we used has a resolution of 0.7 cm-1 per pixel, which means that the position of the band maximum can be determined within (0.7 cm-1. The accuracy of bandwidth measurements is less, and we estimate that it is 1.5 cm-1. Since the total frequency shift is about 2 cm-1 and during the phase transition the bandwidth changes by slightly more than 2 cm-1, the accuracy of the phase transition measurements may be questioned. Due the fact that the intensity is not subject to the resolution limitation and the

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Figure 3. Bandwidth of the 1170 cm-1 peak, the benzene ring stretch mode, of PAA in the bulk phase (solid symbols) and in a gel of 20 nm pore diameter (open symbols).

Figure 4. Temperature dependence of the 1170 cm-1 band position of PAA in the bulk phase (solid symbols) and in a gel of 20 nm average pore diameter (open symbols).

Figure 5. Temperature dependence of the benzene ring-oxygen stretching mode band position for PAA in the bulk phase (solid symbols) and in a gel of 20 nm average pore diameter (open symbols).

error in the R values is below 15%, the transition point determination using this technique is more reliable. Consequently, in further discussion we will apply this approach to determine the phase transition temperature. In the pores, the temperature range for each transition becomes much broader than in the bulk phase, and the exact

Shao and Zerda

Figure 6. Peak intensity ratio, R ) I1244/I1272, of PAA in pores of different sizes: solid circles, bulk PAA; open circles, PAA in 20 nm pores; solid triangles, PAA in 16 nm pores; open triangles, PAA in 8.9 nm pores.

Figure 7. Determination of the solid-nematic transition temperature from Raman peak intensity ratio, R, for PAA in a gel of 20 nm average pore diameter.

determination of a transition point is very difficult. To avoid confusion and reduce the error, we applied a mid-transitionpoint method. Figure 7 shows a sample dependence obtained using the peaks ratio R ) I1244/I1272 for PAA in 20 nm pores. The three sections correspond to the solid state, the transition stage, and the nematic state. Three straight lines are depicted to fit corresponding sections, and they intersect at points A and B. As a convention, point A is the transition onset point, B is the ending point, and the mid-transition-point M is what we use to define the transition point for the PAA in small pores. The error of that procedure depends primarily on the scattering in R values and therefore changes from sample to sample. Typically the error was less than 2 °C, and the maximum value never exceeded 3 °C. Raman spectra of PAA in porous sol-gel glass of modified surfaces are shown in Figure 8 (and for a wider frequency range in Figure 8s). Intensity ratios R ) I1244/I1272 for modified and unmodified surfaces are depicted in Figures 9 and 10 for pores 20 and 16 nm in diameter, respectively. The DSC measurements require crushed samples; therefore, it was impossible to avoid having some PAA outside of the sol-gel pores. For this reason, all DSC curves for impregnated sol-gel samples showed small peaks associated with the I-N and N-S bulk phase transitions. But the much larger peaks were due to the transitions inside the pores. They were shifted

Liquid Crystal PAA

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Figure 8. Raman spectra of PAA in a sol-gel glass of modified surfaces: A, average pore diameter 20 nm; B, average pore diameter 16 nm.

Figure 9. Intensity ratio, R ) I1244/I1272, of PAA in unmodified (solid symbols) and modified (open symbols) gels with 20 nm average pore diameter.

with respect to the bulk phase, but were much wider, and the transition temperatures were determined using the previously described midpoint technique. Figures 11 and 12 illustrate the shift of the N-S transition temperature plotted against 1/R. The dependence is linear, and in smaller pores the transition is observed at lower temperatures. Discussion Band Broadening. When the temperature passes the melting point, the 1270 cm-1 band gradually broadens from 12.5 to 14 cm-1 at 453 K. To explain this effect, we need to discuss briefly mechanisms responsible for the bandwidth. For Raman bands, the intensity is a convolution of rotational and vibrational

Figure 10. Intensity ratio, R ) I1244/I1272, of PAA in unmodified (solid symbols) and modified (open symbols) gels with 16 nm average pore diameter.

contributions, and the bandwidth, ∆ω1/2, is given by

∆ω1/2 ) 1/(cτrot) + 1/(cτvib)

(2)

where τrot and τvib are rotational and vibrational correlation times, respectively. Usually it is assumed that the contribution due to vibrational relaxation in a liquid is temperature independent. According to Fontana,26 for PAA this assumption can be extended to the nematic phase. Although this assumption might not be true for all the modes of all liquid crystal molecules, it is certainly correct for the benzene ring stretching mode of PAA centered at 1170 cm-1. Therefore, the decrease of that bandwidth is mainly due to rotational relaxation. In general, both spinning and tumbling rotational motions of the molecular axis contribute to the band shape. For PAA, due

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Figure 11. Temperature depression obtained from the DSC measurements for the nematic-solid phase transition for PAA in pores of different sizes. The solid symbols are for unmodified samples, and the open symbols are for the gel of modified surface. The line represents the expected linear dependence between (T - To) and R-1.

Figure 12. Comparison of temperature depression of the nematic to solid phase transition for PAA obtained from Raman (squares) and DSC (circles) measurements. The horizontal bars represent errors in determining pore radii from the nitrogen adsorption study (the BET technique). The vertical bars indicate errors in ∆T measurements using Raman or DSC technique.

to large differences in inertial moments along the main molecular axis and directions perpendicular to that axis, the spinning relaxation time is expected to be much faster than that due to the tumbling motion. By choosing the appropriate vibration it is possible to probe only one of those motions.2 For the benzene ring stretching vibration, located at 1170 cm-1, the oscillating dipole moment lies within the plane of the benzene ring and consequently at a small angle with respect to the main molecular axis. Thus, this band probes mainly the tumbling motion, and the contribution due to the spinning relaxation is small. Because of the relatively small moment of inertia associated with the spinning, this motion is expected to be less temperature sensitive than the tumbling. Continuous decrease in the bandwidth on going from 453 to 409 K (the melting point) is thus attributed to progressively slower tumbling motion. When the nematic phase is formed, the tumbling motion completely freezes due to steric restrictions, and only the spinning motion is allowed. As a result, in the nematic phase the bandwidth may be considered constant, because the contribution of the spinning component is temperature independent. When the temperature is lowered below 391 K, the solid phase is formed and the bandwidth of the 1170 cm-1 band decreases by about 2 wavenumbers. This drop can be attributed partially to the abrupt end of the spinning motion and partially to the different vibrational relaxation processes in the solid

Shao and Zerda phase. If we assume that the reduction in the bandwidth came completely from the spinning motion, a reasonable spinning relaxation time of 0.2 ps can be derived from the moment of inertia and temperature. This value is in good agreement with the relaxation times obtained from NMR.27 In the solid phase, the temperature dependence of the bandwidth is complicated due to different intermolecular interactions and couplings that affect vibrational relaxation. The discussion of these processes is beyond the scope of this work. More accurate data on rotational relaxation could be obtained by analyzing the time dependence of the rotational correlation function as we have done for cyclohexane.10 However, to calculate the correlation function, a band must be isolated from the neighbors, so the band intensity in the far wings could be well determined. For PAA this requirement restricts the choices to only one peak centered at 909 cm-1. The intensity of this peak is low; moreover, the interpretation of the correlation function calculated from the 909 cm-1 band shape is very difficult because both the spinning and the tumbling motions contribute to the experimental rotational correlation function. The bandwidths for molecules inside the pores are broader than for molecules in the bulk phase. To discuss this broadening, we need to take into account the amorphous structure and both vibrational and rotational relaxation processes. The amorphous structure is formed in the surface layer as discussed in the study of cyclohexane and causes the Raman band of the low-temperature state to resemble that of the high-temperature state. Since the broadening effects for PAA inside the pores were small, we were able to extend our discussion to the changes in bandwidth. The contribution from vibrational relaxation makes the band broader due to additional interactions between PAA and silica surface which result in faster vibrational relaxation. However, the change in ∆ω1/2 at the transition point should be attributed to the lack of spinning motion in the solid form. It is seen from Figure 3 that the bandwidth measurements can be used to verify the solid-nematic transition in the pores. Small changes in the bandwidth of the 1170 cm-1 band with decreasing temperature have been explained in terms of reduced contribution of the tumbling motion to the overall bandwidth. In the small pores, the tumbling motion is highly restricted so that the bandwidth of the 1170 cm-1 peak remains the same in the liquid and nematic phases. Frequency Shift. Spectral changes during the nematic to solid phase transition are more pronounced than those taking place during the melting transition. Several peaks shift their positions by 2 or more wavenumbers; two examples are shown in Figures 4 and 5. These shifts indicate that the molecular environment in the solid phase is very different from that in the nematic phase. When a line is shifted toward lower frequencies (red shift), it is indicated that the role of attractive interactions in the intermolecular potential is increased.27 A blue shift indicates an increase in the repulsive interactions. Both shifts, red and blue, have been observed in PAA spectra, a clear indication that different parts of this large molecule are exposed to different intermolecular potentials. Although various bands may be shifted in different directions, the onset points of these shifts could be applied to determine the transition temperature. Indeed, Figures 4 and 5 show shifts in opposite directions, but they both occur at the same temperature. This temperature (To ) 392 K) corresponds well to the temperature obtained from the DSC measurements (To ) 391 K). An examination of these figures reveals that the behavior of PAA in the gels is different from those in the bulk phase. For example, in the bulk phase the peak position of the 1170 cm-1

Liquid Crystal PAA band undergoes a 2 cm-1 red shift, while in the gels the magnitude of the frequency jump is smaller (1.5 cm-1 in 20 nm pores, and almost zero in 8.9 nm pores). A smaller frequency shift means that inside the pores the intermolecular potential probed by the benzene ring is not altered significantly when the system changes from the nematic to the solid phase. Relative Band Intensity. The spectral region 1200-1300 cm-1 shows a drastic change when PAA changes from the nematic to the solid phases. As stated before, no changes are observed in the I-S transition. The spectrum of the solid PAA has three components centered at 1244, 1251, and 1272 cm-1. At 391 K the two low-frequency components merge, and in the nematic phase only two bands are present, one at 1244 cm-1 and another at 1274 cm-1 (the band blue shifted by 2 cm-1). The integrated intensity of the high-frequency component does not vary in the discussed temperature range. The spectrum behavior in the region 1200-1300 cm-1 can be explained as follows. In the solid phase PAA molecules are at fixed positions with the long axes aligned approximately perpendicular to the (100) plane.28 They form structures with the benzene rings of the two neighboring molecules facing each other or perpendicular to each other. There are four molecules per unit cell, and the correlation between orientations of the molecules is responsible for the 1244 cm-1 band splitting. In the nematic structure molecules undergo spinning motion and no preferred orientation is possible. Phenyl rings are highly polarizable, and the strong dipolar forces in the solid phase may affect the transition dipole moment responsible for the band intensity. In the nematic phase this interaction reduces sharply as the molecules acquire freedom to translate and to spin. As a result, the splitting is gone and the band intensity is diminished. In Figures 11 and 12, the shift of the solid-nematic phase transition temperature is depicted as a function of the reciprocal of the pore size (1/Rm). The shift is defined as the difference between the transition temperatures of the trapped phase T and the bulk phase To. The data in Figure 12 were obtained independently from the Raman intensity ratio and the DSC measurements. The data obtained from the analysis of the frequency shift of the 1170 cm-1 or the 1272 cm-1 peaks showed a similar trend but are not shown in this figure. From Figure 12 larger temperature depressions are observed for pores of smaller diameter. This result confirms the prediction of the capillary condensation theory,29 computer simulation studies,30-32 and experimental results.9 Surface Effects. To separate the geometrical effects from surface interactions, PAA molecules were impregnated into gel samples of modified surface. Surface modification reduced the pore diameter by 0.15-0.20 nm.7 Comparing this value with the size of the pore, 8.9-20 nm, we assumed that surfacemodified samples had pore sizes identical to those of the untreated samples. In the samples of modified surfaces, the hydroxyls are replaced by trimethylsilyl (-Si-(CH3)3) groups. The new groups are less polar and reduce the attractive interactions between the dopant molecules and the silica surface, as shown by Nikiel et al.2 and Koone et al.7 Raman spectra of PAA in the 20 and 16 nm modified gels are shown in Figure 8, and in a broader frequency range in Figure 8s. The spectra of PAA in gels of modified and unmodified surfaces differ only slightly. By analyzing the temperature dependence of the I1244/ I1272 intensity ratio (Figures 9 and 10), we conclude that the N-S transition takes place at a slightly higher temperature for the unmodified surfaces in the large pores, but the differences diminish with decreasing pore size. For small pores the

J. Phys. Chem. B, Vol. 102, No. 18, 1998 3393 geometrical effect predominates and no differences can be observed. The results are summarized in Figure 11. Conclusions Restricted geometry effects on phase transitions were studied in this work. Phase transitions of PAA were investigated using Raman spectroscopy and the DSC technique. Surfacemolecule interaction effects were also studied by using solgel glass with modified pore surfaces. We found that the phase transition temperatures of molecular systems inside small pores are depressed, and the depressions are pore size dependent. This phenomenon was clearly observed for the solid to nematic phase transition of PAA. From the DSC and the Raman band intensity measurements, we quantitatively determined the temperature shifts of the transition points, and for the pores of average diameter 8.9 nm the depressions were 33 °C below the corresponding bulk phase transformation. The results showed that the temperature depressions were linearly dependent on the reciprocal of the pore diameter. This fact verified the prediction of the capillary condensation theory. The phase transition temperatures were found to extend over a wide temperature range. This effect can be associated with the distribution of pore diameters. In larger pores the phase transition takes place at higher temperatures than in smaller pores, resulting in a broad temperature distribution. However, it is also possible that this effect is associated with another phenomenon. In the bulk phase the transitions of PAA are of first order, as indicated by the abrupt changes in order parameters, Raman intensity band ratio, bandwidth, and maximum position. In the pores, the transitions were diffused over a wide temperature range and resembled the second-order phase transitions. This observation confirms conclusions of previous studies on phase transitions of various materials in small geometries.12-14 Surface modification had a profound effect on the phase transitions of cyclohexane, particularly on the cubic to monoclinic transformation. For PAA the surface modification had much less influence mainly because of stronger intermolecular couplings. For molecular systems with strong intermolecular couplings, the boundary condition change due to the replacement of -OH groups by -Si(CH3)3 is not sufficient to modify the phase behavior, and it is the restricted space that limits the size of a crystal and dominates the changes of phase transitions. Supporting Information Available: Six figures with Raman spectra of PAA in the frequency range between 1000 and 1650 cm-1 recorded at different temperatures: Figure 1s, the bulk phase; Figure 2s, PAA inside porous gels of average pore diameters 20, 16, and 8.9 nm; Figure 8s, PAA inside gels of modified surfaces (4 pages). Ordering information is given on any current masthead page. References and Notes (1) Awschalom, D. D.; Warnock, J. Phys. ReV. B 1987, 35 (13), 6779. (2) Nikiel, L.; Hopkins, B.; Zerda, T. W. J. Phys. Chem. 1990, 94, 7458. (3) Nikiel, L.; Zerda, T. W. J. Phys. Chem. 1990, 93, 8464. (4) Dore, J. C.; Dunn, M.; Hasebe, T.; Strange, J. H. Colloid Surf. 1989, 36, 199. (5) Gorbatschow, W.; Arndt, M. Stannarius, R.; Kramer, M. Europhys. Lett. 1996, 35, 719. (6) Korb, J. P.; Malier, L.; Cros, F.; Xu, S.; Jonas, J. Phys. ReV. Lett. 1996, 77, 2312. (7) Koone, N.; Shao, Y.; Zerda, T. W. J. Phys. Chem. 1995, 99, 16976. (8) Sieminska, L.; Zerda, T. W. J. Phys. Chem. 1996, 100, 4591. (9) Jackson, C. L.; McKenna, G. B. J. Chem. Phys. 1990, 93, 9002.

3394 J. Phys. Chem. B, Vol. 102, No. 18, 1998 (10) Shao, Y.; Hoang, G.; Zerda, T. W. J. Non-Crystal. Solids 1995, 182, 309. (11) Dadmun, M. D.; Muthukumar, M. J. Chem. Phys. 1993, 98, 4850. (12) Cramer, C.; Cramer, T.; Stannarius, R. J. Chem. Phys. 1997, 106, 3730. (13) Qian, S.; Iannacchione, G. S.; Finotello, D. Phys. ReV. E 1996, 53, R4291. (14) Iannacchione, G. S.; Finotello, D. Phys. ReV. E 1994, 50, 4780. (15) Sheng, P. Phys. ReV. A 1982, 26, 1610. (16) Grinberg, F.; Kimmich, R. J. Chem. Phys. 1996, 105, 3301. (17) Miyano, K. Phys. ReV. Lett. 1979, 43, 51. (18) Brinker, C. J.; Scherer, G. W. Sol-Gel Science; Academic Press: New York, 1990. (19) Hench, L. L.; Wang, S. H.; Nogues, J. L. Gel Silica Optics. In Multifunctional Materials; Gunshor, R. L., Ed.; SPIE: Bellingham, WA, 1988; Vol. 878, pp 76-85. (20) Sindolf, D.; Maciel, G. E. J. Phys. Chem. 1983, 87, 5516. (21) Nikiel, L.; Zerda, T. W. J. Phys. Chem. 1991, 95, 4063.

Shao and Zerda (22) Freyman, R.; Servant, R. Ann. Phys. 1945, 20, 131. (23) Maier, W.; Englert, G. Z. Elektrochem. 1959, 62, 1020; Z. Physik. Chem. 1959, 19, 168. (24) Amer, N. M.; Shen, Y. R. J. Chem. Phys. 1972, 56, 2654. (25) Kirov, N.; Simova, P. Vibrational Spectroscopy of Liquid Crystals; Bulgarian Academy of Sciences, 1984. (26) Fontana, M. P. Raman, Infrared and Fluorescence Spectroscopy of Liquid Crystals. Physics of Liquid Crystalline Materials; Khoo, I. C., Simoni, F., Eds.; Gordon & Breach Science: Philadelphia, 1988. (27) Benson, A. M., Jr.; Drickamer, H. G. J. Chem. Phys. 1957, 27, 1164. (28) Krigbaum, W. R.; Chatani, Y.; Barber, P. G. Acta Crystallogr. B 1970, 26, 97. (29) Defay, R.; Prigogine, I.; Bellemans, A.; Everett, D. H. Surface Tension and Adsorption; Longmans: 1966. (30) Brodka, A.; Zerda, T. W. J. Chem. Phys. 1992, 97, 8464. (31) Brodka, A.; Zerda, T. W. J. Chem. Phys. 1996, 104, 6319.